1. Field of the Invention
This invention relates to an LED lamp apparatus such as a rear combination lamp apparatus and an LED head lamp apparatus for vehicles, which controls an LED lamp for vehicles using light emitting diodes as a light source.
In this specification, an LED chip per se is referred to as “light emitting device,” and the whole system including an LED chip-mounted package resin or lens system or other optical system is referred to as “light emitting diode” or “LED.”
2. Related Art
An increase in brightness of light emitting devices has led to extensive use of an LED lamp such as a rear combination lamp and an LED head lamp for vehicles using LEDs as a light source, for example, in stop lamps of automobiles. LEDs have a narrow-band emission spectrum, and the emitted light has good visibility. Further, the speed of response of LED is so high that the speed of transmission of signals to following vehicles is high and, thus, the distance necessary for stopping the travel of vehicles during high-speed traveling can be significantly reduced. In addition, since LEDs per se are a monochromatic light source, unlike incandescent lamps, there is no need to cut off light rays other than light of necessary color through a filter, and monochromatic light sources with a high emission efficiency can be realized. This also leads to energy saving.
An example of this type of an LED lamp apparatus for vehicles is shown in FIG. 1. Specifically, FIG. 1 is a circuit diagram showing an example of a conventional LED lamp apparatus for vehicles. As shown in FIG. 1, this LED lamp apparatus 50 for vehicles comprises a control unit 51 and a light emitting diode unit 52. Eight light emitting diodes 54 constituting the light emitting diode unit 52 are connected in parallel with one another.
Therefore, in the conventional LED lamp apparatus for vehicles, a current limiting circuit (a resistor or a transistor) 55 should be provided for each of the light emitting diodes 54. Increasing the number of the light emitting diodes 54 requires increasing the number of the current limiting circuits 55. This renders the heating value of the current limiting circuits 55 per se unnegligible. Further, the number of wirings necessary for connecting the control unit 51 to the light emitting diode unit 52 (power supply line+number of parallel light emitting diodes) is increased. This necessitates troublesome wiring work and incurs higher cost. Furthermore, a variation in forward voltage among the light emitting diodes 54 causes a variation in flowing current which in turn results in a variation in emitted light brightness among the light emitting diodes 54. An additional problem of the conventional technique is that, when the current limiting circuit 55 is a resistor, a fluctuation in power supply voltage causes a change in current, which flows into the light emitting diodes 54, which in turn results in a fluctuation in brightness. Furthermore, another problem of the conventional technique is as follows. In general, the voltage of a battery in an automobile is about 12 V, and the forward voltage of a light emitting diode is about 2 V. Therefore, when the arrangement of LEDs in series is contemplated for lighting, the upper limit of the number of LEDs is 6.